Process for selective grinding of coal

ABSTRACT

A process for preparing coal for use as a fuel. Forming a coal-water slurry having solid coal particles with a particle size not exceeding about 80 microns, transferring the coal-water slurry to a solid bowl centrifuge, and operating same to classify the ground coal-water slurry to provide a centrate containing solid particles with a particle size distribution of from about 5 microns to about 20 microns and a centrifuge cake of solids having a particle size distribution of from about 10 microns to about 80 microns. The classifer cake is reground and mixed with fresh feed to the solid bowl centrifuge for additional classification.

CONTRACTUAL ORIGIN OF THE INVENTION

The U.S. Government has rights in this invention pursuant to ContractNo. DE-AC22-87PC79867 between the U.S. Department of Energy and BechtelNational, Inc.

BACKGROUND OF THE INVENTION

This invention relates to a method or process of selectively grindingcoal to liberate mineral and ash matter entrained with coal and toprovide a particle distribution which is in a well defined andrelatively narrow range. Modern coal mining and coal cleaning techniquesare generating increasing quantities of degraded coal materials. Coalpreparation plants produce large quantities of crushed coal and refusewith high water or moisture content. There has been a substantial effortby the Department of Energy to determine effective methods of cleaningcoal, of agglomerating the coal into pellets using effective binders.One such agglomeration process is disclosed in U.S. Pat. No. 4,615,712issued to Wen, Oct. 7, 1986 and assigned to the United States of Americaas represented by the U.S. Department of Energy, the disclosure of whichis incorporated herein by reference.

Traditionally, coal crushed to a size below 150 or even 50 mm has beenphysically cleaned and is substantially reduced of minerals such aspyrite and ash. Generally, advanced cleaning may be accomplished byphysically grinding coal to separate the mineral and ash from the coalparticles. Theoretically, fine grinding of coal will result insubstantially all of the ash and minerals such as pyrites being free ofcoal particles.

A problem inherent in grinding of coal is that a portion of the coal inany grinding apparatus is ground to a finer degree than other portionsof the coal. For instance, coal passing through a 200 mesh screen maytypically contain various particle sizes up to about 80 microns. Theparticle size distribution may vary considerably but it will be thelarger particles for the most part, which contain both pyrite and coalas well as coal and ash. Inefficiencies are introduced in the system byvirtue of regrinding coal which is already ash free or pyrite free orcoal which has particles in the desired range. Regrinding is expensiveand results in particles which are finer than desired and are moredifficult to work with later in the process.

Physical coal cleaning involve two distinct steps. The initial sizereduction step prepares the coal by crushing, grinding or micronizing inorder to liberate the ash and the pyrite. Size reduction by physicalcrushing produces a mixture consisting of discrete particles of low ashcoal and particles high in ash or pyrite. Thereafter, the mixture ofdiscrete particles of low ash coal and the particles with either highash or pyrite are segregated into clean coal and into refuse product.The separation step may be accomplished by specific gravity separation,froth flotation methods or by agglomeration methods which are known anddo not form part of this invention. This invention relates to animproved and more efficient grinding process for producing coalparticles having a size distribution in the range of from about 5microns to about 20 microns and which produces particles which aresubstantially free of minerals such as pyrites and ash, withoutexcessive grinding.

Heretofore, reduction of coal to micron sizes has been accomplished withan attrition mill which consists of a cylindrical vessel fitted with anagitator and filled with a hard grinding media such as ceramic or ironshot. The feed consisting of a coal slurry and water containing bothlarge and small particles some of which are high in ash and some ofwhich are high in pyrite content is introduced at one end of the vessel.As the agitator turns, the grinding media disintegrates the particles ofcoal in the feed slurry by attrition. The ground slurry exits at theopposite side of the vessel which is fitted with a screen to prevent thegrinding media leaving the mill with the slurry.

The single pass agitator mill method of crushing or grinding coal isunsatisfactory because the ground product may have an average particlesize which is acceptable, say in the 20 microns range, but the particlesthemselves may have a wide size distribution from submicron size to ashigh as 160 microns. Examination of the particles shows that the finestparticle sizes are predominantly made up of ash-free coal andpyrite-free coal, while the coarser sizes contain coal with high ash andpyrite contents. The characteristics of the ground product fromattrition mills reduces the efficiency of the micronizing and cleaningoperations as well as the dewatering and water treatment steps.Overgrinding increases the difficulty of recovering the products,dewatering the products and clarifying the water for environmentalpurposes. And, if the high ash and high pyrite-containing coal particlesare to be reduced to the required size in a single pass, the entire feedstock must be ground even more finely thereby not only increasing thegrinding cost but also aggravating the problems aforesaid.

SUMMARY OF THE INVENTION

Accordingly, it is a principal object of the present invention toprovide a process of achieving the required level of liberating ash andminerals such as pyrites from coal feed stock with a minimum sizereduction.

Another object of the invention is to provide a process for micronizingcoal which provides a product having relatively narrow size distributionand in which both over and under micronizing are avoided.

Another object of the invention is to provide a process for preparingcoal for use as a fuel, comprising grinding coal particles in thepresence of water to form a coal-water slurry having solid coalparticles with a particle size not exceeding about 80 microns,classifying the ground coal-water slurry in a solid bowl centrifuge toprovide a centrate containing solid particles with a particle sizedistribution of from about 5 microns to about 20 microns and to providea centrifuge cake of solids having a particle size distribution fromabout 10 microns to about 80 microns, and regrinding the classifiercake.

A final object of the invention is to provide a continuous process forpreparing coal for use as a fuel, comprising grinding coal particles inthe presence of water to form a coal-water slurry having solid coalparticles present in an amount of about 30% by weight with a particlesize not exceeding about 80 microns, classifying the ground coal-waterslurry in a solid bowl centrifuge to provide a centrate containing solidparticles present in a concentration greater than about 10% by weightwith a particle size distribution not to exceed about 20 microns and toprovide a centrifuge cake of solids having a particle size distributionof from about 10 microns to about 80 microns, and regrinding thecentrifuge cake, and introducing the reground cake having particle sizesnot exceeding about 50 microns and water with fresh feed to the solidbowl centrifuge for classification to produce a centrate having solidswith a particle size distribution not to exceed about 20 microns.

The invention consists of certain novel features and a combination ofparts hereinafter fully described, illustrated in the accompanyingdrawings, and particularly pointed out in the appended claims, it beingunderstood that various changes in the details may be made withoutdeparting from the spirit, or sacrificing any of the advantages of thepresent invention.

BRIEF DESCRIPTION OF THE DRAWINGS

For the purpose of facilitating an understanding of the invention, thereis illustrated in the accompanying drawings a testing arrangementthereof, from an inspection of which, when considered in connection withthe following description, the invention, its construction andoperation, and many of its advantages should be readily understood andappreciated.

FIG. 1 is a flow diagram of a process for selectively grinding coal.

DESCRIPTION OF THE PREFERRED EMBODIMENT

A source of coal 10 is connected by a conveyor 11 to a source of water12 which forms a slurry in line 13 and is introduced to a rod mill 15 atone end thereof. The outlet from the rod mill 15 leads to an accumulator17 and therefrom to a tank 18, the outlet 19 of which is connected to asolid bowl centrifuge 20. A scale 21 and suitable valve 22 are providedto establish or interrupt a measurable communication between thematerial in the vessel 18 and the centrifuge 20. The centrifuge 20 maybe one of several solid bowl centrifuges commercially available such asby Bird Machine Co. The centrifuge 20 is operated as a classifier whichseparates particles having a size distribution in the range of fromsubmicron size up to about 20 microns size which exits the centrifuge 20via a line 25 to a collection vessel 26 for the centrate. Depending onthe operation characteristics of the classifier or centrifuge 20 thesolids contents in the centrate may range from about 5% to upwards of15% by weight. The centrifuge cake 27 exits from the centrifuge viaoutlet 28 and may be reformed as slurry by the introduction of waterfrom a source of water 29 and transferred through a pipe 30 to a beadmill 35 through a feed bin 34. The bead mill 35 is a grinding apparatuscapable of grinding to finer particle sizes for instance less than about50 microns than is the rod mill 15 initially used in the grindingprocess. The reground centrifuge cake leaves the bead mill 35 via anoutlet 36 and is transported to the vessel 18 and hence to the solidbowl centrifuge classifier 20.

By this method, it is possible to avoid over grinding coal particleswhich are already substantially ash free and pyrite free because theselection process depends principally on centrifugal force. Thecentrifugal force will separate both as to particle size and as toweight. With respect to particle size, it has been found as describedabove, that smaller particles tend to be more ash free and more mineraland pyrite free. Particles high in pyrite content tend to be heavier andso that the heavier particles and the larger particles tend to besegregated and recycled to the bead mill 35 for additional grinding.This method accordingly separates the particles which are substantiallymineral free and ash free from those particles which need additionalgrinding to clean the particles, thereby preventing regrinding ofparticles which are already clean. The centrate 26 is thereaftertransported to a suitable cleaning facility (not shown) which willseparate the tailings including coal of high ash content and mineralparticles from the clean coal. The clean coal is thereafter treated andformed into the appropriate fuel for transport and storage and then forcombustion. The feed size distribution for the two types of coal used inthe various examples are set forth in Table 1.

In testing the inventive process, a solid bowl centrifuge, model 0250was obtained from Bird Machine Company and two types of ground coal weremade available. One coal was from Illinois No. 6 seam and the other wasa Splint coal. All coal was ground in the rod mill 15 to a nominal sizeof minus 200 mesh, that is approximately 80% of the coal had particlediameters of about 76 microns or less. The centrifuge 20 was operable aseither a decanter or a classifier depending on operationcharacteristics.

The Splint coal was used to determine the best settings of thecentrifuge 20 for the classification approach. The variables were flowrate, solid concentration and depth of the pool within the centrifuge.Table 2 lists the above parameters for each of the examples S1-S9.Samples were taken and analyzed for particle size distribution, percentsolids and ash and sulfur content; Table 3 shows the results.

                  TABLE 1                                                         ______________________________________                                        FEED SIZE DISTRIBUTION                                                        PART-                                                                         ICLE   SPLINT           ILLINOIS NO. 6                                        SIZE   BALL MILL PRODUCT                                                                              BALL MILL PRODUCT                                     MI-            CUMULATIVE         CUMULATIVE                                  CRONS  WT %    WT %         WT %  WT %                                        ______________________________________                                         <6    11.1    11.1         10.7  10.7                                        6-8    5.4     16.5         3.4   14.1                                         8-10  3.3     19.8         2.7   16.8                                        10-15  8.1     27.9         8.4   25.2                                        15-20  7.3     35.2         6.9   32.1                                        20-30  15.3    50.5         16.7  48.8                                        30-38  11.2    61.7         11.2  60.0                                        38-75  18.6    80.3         23.9  83.9                                        >75    19.7    100.0        16.1  100.0                                              100.0                100.0                                             ______________________________________                                    

                  TABLE 2                                                         ______________________________________                                        SET-UP TEST CONDITIONS                                                                 FEED RATE   POOL SETTING  SOLIDS                                     TEST NO. gpm         mm            WT %                                       ______________________________________                                        S 1      3           145-DEEP      30                                         S 2      6           145           30                                         S 3      9           145           30                                         S 4      9           150           30                                         S 5      6           150           30                                         S 6      9           170           30                                         S 7      9           175           30                                         S 8      9           175           20                                         S 9      12          175-SHALLOW   20                                         ______________________________________                                    

                                      TABLE 3                                     __________________________________________________________________________    SET-UP TEST RESULTS                                                                        WT % PASSING (MICRONS)                                                                             WT % WT %                                   TEST NO                                                                             PRODUCT                                                                              75 38 30 20 15 10 6  SOLIDS                                                                             ASH                                    __________________________________________________________________________    S 1   CAKE   81.4                                                                             61.0                                                                             49.0           58.6 4.1                                          CENTRATE                                                                             100                                                                              100                                                                              100            1.2  14.1                                   S 2   CAKE   79.5                                                                             58.0                                                                             49.4           56.2 4.0                                          CENTRATE                                                                             100                                                                              100                                                                              100            4.4  7.2                                    S 3   CAKE   81.9                                                                             58.6                                                                             52.6           56.4 4.0                                          CENTRATE                                                                             100                                                                              100                                                                              99.3           3.1  9.0                                    S 4   CAKE   80.8                                                                             62.2                                                                             53.3                                                                             39.9                                                                             31.4                                                                             22.0                                                                             13.5                                                                             55.7 4.0                                          CENTRATE                                                                             100                                                                              100                                                                              99.7           3.4  6.9                                    S 5   CENTRATE                                                                             100                                                                              99.9                                                                             94.6                                                                             91.4                                                                             87.5                                                                             76.8                                                                             55.4                                                                             8.9  3.7                                    S 6   CENTRATE                                                                             100                                                                              100                                                                              99.5           9.5  4.2                                    S 7   CENTRATE                                                                             100                                                                              99.8                                                                             93.9                                                                             88.8                                                                             80.6                                                                             63.9                                                                             42.5                                                                             11.1 4.4                                    S 8   CENTRATE                                                                             100                                                                              100                                                                              99.8                                                                             99.3                                                                             93.5                                                                             75.3                                                                             48.5                                                                             6.5  5.3                                    S 9   CENTRATE                                                                             99.5                                                                             94.4                                                                             87.5                                                                             73.8                                                                             64.2                                                                             50.1                                                                             33.9                                                                             15.7 3.5                                    __________________________________________________________________________

Samples were taken from the centrifuge cake 27 as well as the centrate26 during examples S1-S4 and later only centrate samples were analyzed.The centrifuge 20 operates as a decanter where there is a low feed rateand deeper pool settings and this is shown in examples S1-S4. After anincrease in the flow rate and a shallower pool setting, the centrifuge20 operates as a classifier which is assisted in operation by loweringthe solids concentration to 20% by weight, it should be noted that theweight percent of ash in Table 3 is on a dry basis. The change from adecanter to a classifier of the centrifuge 20 is indicated by theincrease in solids, including coarser particles, which report to thecentrate 26. This increase is proportional to a decrease in the ashcontent of the solids, an indication that cleaner coal particles arerecovered with the centrate. However, it should be understood that thepurpose of the centrifuge 20 is not to clean the coal in the totalmaterial balance sense but to segregate the clean fine particles fromthe more contaminated larger or more contaminated heavier particleswhich when reground provide additional clean particles. In this way,continued cycling of material through the bead mill 35 and thecentrifuge 20 will eventually provide all materials in the centrate 26,both clean and dirty, but the coal particles will be substantially ashfree and the pyrite particles will have low coal content.

That the process is effective is indicated by a grab sample of the plus30 micron material which had an ash content of 1.5 percent, confirmingthe selective recovery of coarser but cleaner coal with the centrate 26.The initial tests with the Splint coal was used to provide improvedparameters for tests with the Illinois No. 6 coal. The data wereobtained using a pool setting of 175 mm, a solids concentration of 20weight percent and a feed rate of 11 gpm. These parameters were selectedfor the specific model of Bird centrifuge. It is understood that othercentrifuges are available of different sizes, for instance a Sharples orBretby solid bowl centrifuge may be used in the process with capacitiesof up to 60 tons per hour. In such an event, the operating conditions ofthe centrifuge will have to be adjusted so that it operates as aclassifier. These adjustments are within the skill of the art.

The test data from the various runs are disclosed in Tables 4 and 5. Thedata show a successful operation of a solid bowl centrifuge 20 acting asa classifier in the closed circuit which may be continuous or batch withthe fine grinding mill 35 operated at approximately 100% circulatingload. Again, it is within the skill of the art to upscale the process toaccommodate commercial quantities in continuous and steady stateconditions. These data show that it is possible to use a solid bowlcentrifuge 20 for the classification of finally ground coal atapproximately 20 micron size for the largest particles. The data showthat this type of classification enhances the selective recovery ofcoarser and heavier particles to the centrifuge cake 27 with cleanercoal particles and smaller more likely ash- free coal constituents intothe centrate 26. The harder coarse coal, more likely containing mineralparticles and ash report to the centrifuge cake 27 and are subjected toadditional grinding in the bead mill 35. The inventive process improvesliberation of the coal and minerals with reduced effort of grinding andavoids over grinding of clean coal particles resulting in a narrowersize distribution range for feed coal to the coal agglomeration processhereinbefore discussed.

                                      TABLE 4                                     __________________________________________________________________________    FIRST PASS CLASSIFICATION                                                     PARTITION-CURVE                                                               __________________________________________________________________________                    WATER         SOLIDS      SULFUR                              STREAM COAL LB/MIN                                                                            LB/MIN                                                                              YIELD WT %                                                                            WT % ASH WT %                                                                             WT %                                __________________________________________________________________________    FEED   20.5     76.8          21.0 8.6    2.84                                CAKE   18.1     15.4  89.3    54.0 8.0    2.75                                CENTRATE                                                                             2.8      60.8  10.7    4.4  13.1   1.75                                __________________________________________________________________________                        FINE COAL                                                 PARTICLE                                                                             FEED             Yf = 10.7%                                            SIZE       CUMULATIVE   REC. WT %                                                                            CUMULATIVE                                                                             COARSE COAL                           MICRON WT %                                                                              WT %     WT %                                                                              5      REC. WT %                                                                              WT %                                  1      2   3        4   Yf *(4)                                                                              6        7                                     __________________________________________________________________________     <6    10.3                                                                              10.3     13.4                                                                              1.4    1.4      6.9                                    6-10  6.4 16.7     20.6                                                                              2.2    3.6      4.4                                   10-15  8.5 25.2     28.0                                                                              3.0    6.6      7.8                                   15-20  6.8 32.0     15.3                                                                              1.6    8.3      8.3                                   20-30  16.8                                                                              48.8     17.9                                                                              1.9    10.2     16.2                                  30-38  11.2                                                                              60.0     4.8 0.5    10.7     12.6                                  38-75  23.9                                                                              83.9     0.0 0.0    10.7     29.9                                  >75    16.1                                                                              100.0    0.0 0.0    10.7     13.9                                  TOTAL  100.0        100.0                                                                             10.7            100.0                                 __________________________________________________________________________           COARSE COAL                        PARTITION                           PARTICLE                                                                             Yc = 89.3%                                                                           CUMULA-                                                                              CALCULATED FEED                                                                             MEDIAN NUMBER                              SIZE   REC. WT %                                                                            TIVE   WT % CUMULATIVE                                                                             PARTICLE                                                                             *100                                MICRON 8      REC. WT %                                                                            10   WT %     SIZE   13                                  1      Yc *(7)                                                                              9      (5) + (8)                                                                          11       12     (8)/(10)                            __________________________________________________________________________    <6     6.2    6.2    7.6  7.6      3.0    81.1                                 6-10  3.9    10.1   6.1  13.7     8.0    64.1                                10-15  7.0    17.1   10.0 23.7     12.5   69.9                                15-20  7.4    24.5   9.0  32.7     17.5   81.9                                20-30  14.5   38.9   16.4 49.1     25.0   88.3                                30-38  11.3   50.2   11.8 60.9     34.0   95.6                                38-75  26.7   76.9   26.7 87.6     56.5   100.0                               >75    12.4   89.3   12.4 100.0    100.0  100.0                               TOTAL  89.3          100.0                                                    __________________________________________________________________________

                  TABLE 5                                                         ______________________________________                                        BEAD MILL GRINDING                                                                    FEED          PRODUCT                                                 PARTICLE           CUMULA-          CUMULA-                                   SIZE               TIVE             TIVE                                      MICRONS   WT %     WT %       WT %  WT %                                      ______________________________________                                         <6       28.2     28.2       47.2  47.2                                       6-10     9.0      37.2       15.2  62.4                                      10-15     6.4      43.6       12.0  74.4                                      15-20     3.0      46.6       9.0   83.4                                      20-30     3.0      49.6       12.0  95.4                                      30-38     15.4     65.0       2.8   98.2                                      38-75     19.9     84.9       1.8   100.0                                     >75       15.1     100.0      0.0   100.0                                     TOTAL     100.0               100.0                                           ______________________________________                                    

While there has been disclosed what is considered to be the testingarrangement of the present invention, it is understood that variouschanges in the details may be made without departing from the spirit, orsacrificing any of the advantages of the present invention.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:
 1. A process for preparingcoal for use as a fuel, comprising grinding coal particles in thepresence of water to form a coal-water slurry having solid coalparticles with a particle size not exceeding about 80 microns,classifying the ground coal-water slurry in a solid bowl centrifuge toprovide a centrate containing solid particles with a particle sizedistribution of from about 5 microns to about 20 microns and to providea centrifuge cake of solids having a particle size distribution of fromabout 10 microns to about 80 microns, and regrinding the classifiercake, wherein more than about 50% of the slurry particles are largerthan 6 microns.
 2. The process of claim 1, wherein the coal-water slurryis continuously prepared.
 3. The process of claim 2, wherein thecentrifuge cake is reground to provide particle sizes not to exceedabout 50 microns.
 4. The process of claim 1, wherein the solidsconcentration in the centrate is greater than about 10% by weight. 5.The process of claim 1, wherein the process is continuous and the solidsconcentration in the centrate is maintained greater than about 10% byweight.
 6. A continuous process for preparing coal for use as a fuel,comprising grinding coal particles in the presence of water to form acoal-water slurry having solid coal particles with a particle size notexceeding about 80 microns, transporting the coal-water slurry to asolid bowl centrifuge, classifying the ground coal-water slurry in asolid bowl centrifuge to provide a centrate containing solid particleswith a particle size distribution of from about 5 microns to about 20microns and to provide a centrifuge cake of solids having a particlesize distribution of from about 10 microns to about 80 microns, andregrinding the classifier cake, and introducing the reground cake withfresh feed to the solid bowl centrifuge for classification to produce acentrate having solids with a particle size distribution of from about 5microns to about 20 microns, wherein more than 50% of the slurryparticles are larger than 6 microns.
 7. The process of claim 6, whereinthe classifier cake is ground in a bead mill and mixed with water beforeintroduction to the classifier.
 8. The process of claim 7, wherein theclassifier cake is ground to produce a size distribution wherein amajority of particles have sizes less than about 20 microns.
 9. Theprocess of claim 1, wherein the cake is continuously introduced to amill for grinding and the reground cake is continuously introduced tothe classifier.
 10. A continuous process for preparing coal for use as afuel, comprising grinding coal particles in the presence of water toform a coal-water slurry having solid coal particles present in anamount of about 30% by weight with a particle size not exceeding about80 microns, classifying the ground coal-water slurry in a solid bowlcentrifuge to provide a centrate containing solid particles present in aconcentration greater than about 10% by weight with a particle sizedistribution not to exceed about 20 microns and to provide a centrifugecake of solids having a particle size distribution of from about 10microns to about 80 microns, and regrinding the classifier cake, andintroducing the reground cake having particle sizes not exceeding about50 microns and water with fresh feed to the solid bowl centrifuge forclassification to produce a centrate having solids with a particle sizedistribution not to exceed about 20 microns, wherein more than 50% ofthe slurry particles are larger than 6 microns.
 11. The continuousprocess of claim 10, wherein the slurry is formed in a rod mill toprovide coal particle sizes not greater than about 80 microns.
 12. Thecontinuous process of claim 10, wherein the centrifuge cake is regroundin a bead mill to provide fine particles to the classifier.
 13. Thecontinuous process of claim 10, and further comprising transferring thecentrate to a separator to separate ash and minerals present from thecoal-water slurry.
 14. The continuous process of claim 10, wherein thecentrate solids concentration is greater than about 15% by weight. 15.The continuous process of claim 6, wherein about 15% of the total of thecentrate solids have particle sizes in the range of from about 6 micronsto about 10 microns.
 16. The continuous process of claim 6, whereinabout 12% of the total of the centrate solids have particle sizes in therange of from about 10 microns to about 15 microns.
 17. The continuousprocess of claim 6, wherein about 9% of the total of the centrate solidshave particle sizes in the range of from about 15 microns to about 20microns.
 18. The continuous process of claim 6, wherein about 12% of thetotal of the centrate solids have particle sizes in the range of fromabout 20 microns to about 30 microns.
 19. The continuous process ofclaim 6, wherein about 35% of the total of the centrate solids haveparticle sizes in the range of from about 6 microns to about 20 microns.20. The continuous process of claim 6, wherein about 50% of the total ofthe centrate solids have particle sizes in the range of from about 6microns to about 30 microns.